A hydrogen sensor is disclosed in Patent Document 1. An alloy used in this hydrogen sensor to store hydrogen is in the form of a thin film. The hydrogen sensor detects storage of hydrogen by making use of the property that the optical reflectivity of the alloy changes as the alloy stores hydrogen.
Such an alloy capable of storing hydrogen can be used not only as a sensor but as an electrode of a hydrogen storage cell. Where the alloy is used as an electrode, it is desirable that the alloy should be able to store as much hydrogen as possible. However, hydrogen storage alloy has its own theoretical value represented by a ratio up to which hydrogen can be stored in the alloy, and it is not generally known that hydrogen storage alloy is capable of storing hydrogen in an amount exceeding its theoretical value.